Polyester film and a production method thereof

ABSTRACT

The object of the present invention is to provide a method for producing a polyester film excellent in rigidity, toughness, dimensional stability, electric properties, etc. and less in thickness fluctuation and surface defects, hence very suitable as a film for various industrial materials such as magnetic recording media, capacitors, heat transfer ribbons and thermal mimeographic stencil paper. The first method of the present invention is a method for producing a polyester film, in which a film made of a resin mainly composed of a polyester is simultaneously biaxially stretched by a simultaneously biaxially stretching tenter oven, comprising the step of effecting small-ratio stretching at an area stretching ratio of 1.0005 to 3.0 times three times or more, to achieve a total area stretching ratio of 25 to 150 times, and the second method of the present invention is a method for producing a polyester film, in which a film made of a resin mainly composed of a polyester is stretched using a simultaneously biaxially stretching tenter oven, comprising the step of effecting a series of operation consisting of stretching and subsequent relaxation twice to less than 10000 times, to achieve a total area stretching ratio of 25 to 150 times.

TECHNICAL FIELD

[0001] The present invention relates to a polyester film highly improvedin film properties and quality compared to conventional polyester films,and a production method thereof.

[0002] More particularly, the present invention relates to a polyesterfilm excellent in rigidity, toughness, dimensional stability, electricproperties, etc. and less in thickness fluctuation and surface defects,hence very suitable as a film for various industrial materials such asmagnetic recording media, capacitors, heat transfer ribbons, thermalmimeographic stencil paper, etc. The present invention also relates to amethod for producing said film.

BACKGROUND ARTS

[0003] Plastic films can be continuously produced as films with largeareas, though this cannot be achieved with other materials. Because oftheir features of high strength, durability, clarity, flexibility andgood surface properties, they are used in various fields needing them inlarge quantities such as magnetic recording media, agriculture,packaging, building materials, etc.

[0004] Above all, biaxially oriented polyester films are used in variousfields since they are excellent in mechanical properties, thermalproperties, electric properties and chemicals resistance, and especiallyas base films for magnetic tapes, they are incomparable to any othermaterials.

[0005] In this field, especially in recent years, the base films arefurther requested to be thinner for meeting the demands forlighter-weight and smaller-sized devices and longer-time recordingcapability. The films for heat transfer ribbons, capacitors and thermalmimeographic stencil paper are also highly requested to be thinner inrecent years.

[0006] However, if a thinner film is produced, the mechanical strengthbecomes insufficient to make the film less stiff and likely to beelongated. So, for example, in the application as a magnetic recordingmedium, the tape is likely to be damaged, or the head touch becomes poorto lower electromagnetic conversion properties. Furthermore, if athinner film is used as a heat transfer ribbon, the ribbon cannot bekept flat during printing, to cause irregular printing or over-transfer.A thinner film used as a capacitor lowers the dielectric breakdownvoltage disadvantageously.

[0007] In the demand for thinner films, films are desired to have higherstrength, by improving mechanical properties such as tensile propertiesincluding Young's modulus.

[0008] So, various methods have been studied to enhance the strengths offilms.

[0009] A generally known method for enhancing the strength of abiaxially oriented polyester film is to re-stretch a once longitudinallyand laterally stretched film in the longitudinal direction for enhancingthe strength in the machine direction as the so-called longitudinalre-stretching method (e.g., Japanese Patent Publication (Kokoku) Nos.42-9270 and 43-3040, Japanese Patent Laid-Open (Kokai) Nos. 46-1119 and46-1120, etc.) For further enhancing the strength also in the transversedirection, it is proposed to re-stretch said longitudinally re-stretchedfilm as the longitudinal re-stretching and lateral re-stretching method(e.g., Japanese Patent Laid-Open (Kokai) Nos. 50-133276 and 55-22915,etc.). Furthermore, it is proposed to once stretch the film in thelongitudinal direction in 2 or more steps and then to stretch in thelateral direction as the multi-step longitudinal stretching method(e.g., Japanese Patent Publication (Kokoku) Nos. 52-33666 and 57-49377,etc.).

[0010] The multi-step longitudinal stretching method is superior to thelongitudinal re-stretching method and the longitudinal re-stretching andlateral re-stretching method in view of higher strength, less filmthickness fluctuation and higher productivity. However, the problem thata film with a higher strength becomes larger also in heat shrinkage andis more frequently broken cannot be solved by the multi-steplongitudinal stretching method either.

[0011] It is also proposed to stretch a film three or more timescontinuously repetitively in at least either the machine direction orthe transverse direction as the small-ratio repetitive stretching method(supermulti-step stretching method) which is one of conventionally knownfilm production methods and is similar to the method of the presentinvention described later (Japanese Patent Laid-Open (Kokai) Nos.8-224777 and 9-57845). However, the inventions described in saidJapanese Patent Laid-Open (Kokai) Nos. 8-224777 and 9-57845 simply showexamples of mainly sequential biaxial stretching, and do not referspecifically to any mechanism, apparatus or process conditions effectivefor simultaneous biaxial stretching. In addition, they do not refer tothe effectiveness of the simultaneous biaxial stretching method by alinear motor system proposed to be used as a preferable apparatus in thepresent invention since it allows high ratio stretching.

[0012] On the other hand, in recent years, linear motor drivensimultaneously biaxially stretching tenter ovens have been developed,and attract attention because of their high film forming speeds, etc.(e.g., Japanese Patent Publication (Kokoku) No. 51-33590, U.S. Pat. Nos.4,853,602 and 4,675,582, etc.).

[0013] The conventional simultaneous biaxial stretching methods such asthe screw method for spreading the clip interval by guiding clips in thegrooves of screws and the pantograph method for spreading the clipinterval using a pantograph have such problems that the film formingspeed is low, that it is not easy to change conditions such asstretching ratio, and that stretching at a high ratio is not easy. Onthe contrary, the linear motor driven simultaneous biaxial stretchingmethod has possibility to solve these problems all at once.

[0014] Said Japanese Patent Publication No. 51-33590 discloses to changethe tenter clip interval by the electric force generated by a linearmotor for allowing highly efficient production. Furthermore, said U.S.Pat. No. 4,853,602 discloses a stretching system using a linear motor,and said U.S. Pat. No. 4,675,582 discloses a system effective forcontrolling many linear motors along the stretching section. However,even these U.S. patents do not refer to the stretching method disclosedin the present invention or the high quality polyester film intended tobe obtained by said method.

[0015] The process conditions for producing a polyester film withexcellent film properties and quality by the linear motor drivensimultaneous biaxial stretching were unknown, and any effectivestretching method was not established yet.

[0016] As described above, the techniques for producing a polyester filmwith high film properties and quality are yet to be improved, and it hasbeen demanded to develop a new technique in this industrial field.

DISCLOSURE OF THE INVENTION

[0017] The object of the present invention is to provide a high qualitypolyester film excellent in rigidity, toughness and dimensionalstability and less in thickness fluctuation and surface defects, and aproduction method thereof.

[0018] The inventors studied intensively any method for improving thefilm properties and quality of polyester films to the extreme extent.

[0019] As a result, it was found that the object of the presentinvention can be achieved by a method for producing a polyester filmusing a simultaneously biaxially stretching tenter oven, comprising thestep of effecting small-ratio stretching at an area stretching ratio of1.0005 to 3.0 times three times or more, to achieve a total areastretching ratio of 25 to 150 times, as a first method of the presentinvention for producing a polyester film.

[0020] It was also found that the object of the present invention canalso be achieved by a method comprising the step of effecting a seriesof operation consisting of stretching and subsequent relaxation twice toless than 10000 times, to achieve a total area stretching ratio of 25 to150 times, as a second method of the present invention for producing apolyester film.

[0021] The first or second method of the present invention was adopted,it was surprisingly found that

[0022] (1) the Young's modulus of a polyester film becomes very large,while the heat shrinkage becomes small,

[0023] (2) the stretching ratio can be raised to enhance theproductivity,

[0024] (3) the film fluctuates less in thickness and is broken lessfrequently, and

[0025] (4) the film is likely to be higher in the degree ofcrystallinity, and even if the temperature of the heat treatment zone islowered, the heat,shrinkage does not increase.

[0026] Thus, the present invention has been completed.

[0027] The gist of the present invention is the method for producing apolyester film described below as the first or second method.

[0028] The first method of the present invention is a method forproducing a polyester film, in which a film made of a resin mainlycomposed of a polyester is stretched by a simultaneously biaxiallystretching tenter oven, comprising the step of effecting small-ratiostretching at an area stretching ratio of 1.0005 to 3.0 times threetimes or more, to achieve a total area stretching ratio of 25 to 150times (hereinafter called this first method of the invention is called“production method (I)”).

[0029] The polyester film production method (I) of the present inventionincludes the following preferable embodiments.

[0030] (a) Effecting the small-ratio stretching continuously three ormore times.

[0031] (b) Repeating the small-ratio stretching 10 to less than 10000times.

[0032] (c) Effecting the small-ratio stretching of a cast film in atemperature range of (glass transition temperature (Tg)+10)° C. to(Tg+120)° C.

[0033] (d) Continuously repeating the small-ratio stretching of a castfilm till the degree of crystallinity of the film reaches 3% to lessthan 30%.

[0034] The second method of the present invention is a method forproducing a polyester film, in which a film made of a resin mainlycomposed of a polyester is stretched using a simultaneously biaxiallystretching tenter oven, comprising the step of effecting a series ofoperation consisting of stretching and subsequent relaxation twice toless than 10000 times, to achieve a total area stretching ratio of 25 to150 times (hereinafter this second method is called “production method(II)”).

[0035] The polyester film production methods (I) and (II) of the presentinvention include the following preferable embodiment.

[0036] (a) The clips are driven by a linear motor system.

[0037] The polyester film of the present invention is produced by saidproduction method (I) or (II) of the present invention.

[0038] The polyester film includes the following preferable embodiments.

[0039] (a) The sum of the Young's modulus in the machine direction ofthe film and that in the transverse direction is 8 to 30 GPa, and thesum of heat shrinkage percentages at 100° C. for 30 minutes is 2% orless.

[0040] (b) The degree of crystallinity is 30 to 90%.

[0041] (c) The polyester is polyethylene terephthalate, polyethylenenaphthalate or their copolymer or modified polymer.

[0042] (d) The inherent viscosity is 0.6 or more.

[0043] The polyester film of the present invention is suitable for suchapplications as magnetic recording media, capacitors, heat transferribbons and thermal mimeographic base films.

[0044] The present invention is described below in more detail.

The Best Embodiments of the Invention

[0045] The polyester referred to in the present invention is a polymercontaining at least 80 wt % of a polymer obtained by polycondensation ofa diol and a dicarboxylic acid. The dicarboxylic acids which can be usedhere typically include terephthalic acid, isophthalic acid, phthalicacid, naphthalenedicarboxylic acid, adipic acid and sebacic acid, andthe diols which can be used here include typically ethylene glycol,trimethylene glycol, tetramethylene glycol and cyclohexane dimethanol.

[0046] The polyesters which can be used here include, for example,polymethylene terephthalate, polyethylene terephthalate, polypropyleneterephthalate, polyethylene isophthalate, polytetramethyleneterephthalate, polyethylene-p-oxybenzoate,poly-1,4-cyclohexylenedimethylene terephthalate andpolyethylene-2,6-naphthalate. These polyesters can be homopolymers andcopolymers, and the comonomers which can be used here include, forexample, diols such as diethylene glycol, neopentyl glycol andpolyalkylene glycols, dicarboxylic acids such as adipic acid, sebacicacid, phthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylicacid, and hydroxycarboxylic acids such as hydroxybenzoic acid and6-hydroxy-2-naphthoic acid.

[0047] In the present invention, especially polyethylene terephthalate,polyethylene naphthalate (polyethylene-2,6-naphthalate), their copolymerand modified polymers are preferable since the effect of the presentinvention can be highly manifested.

[0048] In the present invention, it is preferable that the inherentviscosity of the polyester is 0.6 or more. More preferable is 0.8 ormore and most preferable is 1.0 or more. A polyester with a highmolecular weight usually has a disadvantage that the heat shrinkage ofthe film becomes larger at a higher Young's modulus. However, accordingto the production method of the present invention, since themicrostructure is effectively eased while the total area stretchingratio of the film is enhanced, the heat shrinkage can be lessened.

[0049] In the present invention, “stretching” refers to the operation tomake a film oriented in the machine direction or in the transversedirection by carrying the film while holding its both the edges by clipsusing a simultaneously biaxially stretching tenter oven for pulling itin at least either the machine direction or the transverse direction.The “machine direction” of a film refers to the longitudinal directionof the film, and the “transverse direction” refers to the lateraldirection of the film. The “simultaneous biaxial stretching” refers tothe operation to stretch a film simultaneously in both the machinedirection and the transverse direction. Before and/or after the“simultaneous biaxial stretching”, further other stretching can beeffected. For example, stretching in the transverse direction or themachine direction or sequentially in both the directions can be followedby the simultaneous stretching in the machine direction and thetransverse direction, or the simultaneous biaxial stretching can befollowed by the stretching in the transverse direction or the machinedirection or sequentially in both the directions, within the scope ofthe present invention.

[0050] It can be said preferable to use a linear motor drivensimultaneously biaxially stretching tenter oven in the present inventionas a stretcher capable of freely changing the stretching direction andthe stretching ratio.

[0051] As described before, a linear motor driven simultaneouslybiaxially stretching tenter oven attracts attention since it

[0052] (1) allows the film forming speed and the film width to beincreased to not lower or not smaller than those achieved in theconventional sequential biaxial stretching,

[0053] (2) allows stretching at a high ratio, and

[0054] (3) allows the film deformation pattern to be freely changed inthe steps of stretching, heat treatment and relaxation.

[0055] In the present invention, it is especially preferable to adoptthe supermulti-step stretching using the linear motor drivensimultaneously biaxially stretching tenter oven, for obtaining apolyester film with high film properties and quality at a low cost.

[0056] The production method (I) of the present invention is describedbelow. In this method, it is necessary that the area stretching ratio ofone time of small-ratio stretching in the supermulti-step stretching isset at 1.0005 to 3.0 times, that the small-ratio stretching is effectedthree or more times and that the total area stretching ratio is 25 to150 times.

[0057] The “area stretching ratio” refers to the product obtained bymultiplying the stretching ratio in the machine direction of the film bythe stretching ratio in the transverse direction. In the presentinvention, “small-ratio stretching” refers to stretching at a stretchingratio smaller than usual, generally the stretching in which thestretching ratio is 3.0 times or less as the area stretching ratio.

[0058] In the present invention, the small-ratio stretching as describedabove is repeated. If the area stretching ratio by one time ofsmall-ratio stretching exceeds 3.0 times, the intended effects of thepresent invention are unlikely to be obtained, and if less than 1.0005times, the stretching cannot be a practical requirement.

[0059] It is more preferable that the area stretching ratio by one timeof small-ratio stretching is 1.005 to 2.0 times, and a further morepreferable range is 1.01 to 1.5 times. It is preferable that thefrequency of small-ratio stretching in the production method (I) is 10to less than 10000 times, and a more preferable range is 50 to less than1000 times. Furthermore, it is preferable to effect the small-ratiostretching continuously three or more times. The final total areastretching ratio must be 25 to 150 times. A preferable range is 30 to120 times, and a more preferable range is 50 to 100 times. If the totalarea stretching ratio is less than 25 times, the intended effects of thepresent invention cannot be obtained, and it is practically difficult toachieve a total stretching ratio of more than 150 times.

[0060] The “one time of small-ratio stretching” in the present inventionis defined as

[0061] (1) halting stretching after each time of small-ratio stretchingas shown in the production method (I), or

[0062] (2) a series of continuous operation consisting of stretching andrelaxation as shown in the production method (II), or

[0063] (3) continuous stretching without changing stretching conditionssuch as stretching mode, stretching ratio, temperature and stretchingrate.

[0064] In the case of the above (3), that is, in the case of continuousstretching, the relation between time and the stretching ratio in themachine or transverse direction (or clip speed) can be expressed by astraight line (monotonous increase) or a curve. In the case of a curve,a point of inflection in the curve expressing the relation between timeand the stretching ratio in the machine or transverse direction is anend of each time of small-ratio stretching. The “stretching mode” refersto any of “longitudinal stretching” mode, “lateral stretching” mode and“simultaneous biaxial stretching” mode. In the above (1), whenstretching is halted, it is preferable to halt for a periodcorresponding to {fraction (1/100)} to ⅘ of the time taken forstretching. A more preferable stretching halting period is {fraction(1/100)} to ⅗ of the time taken for the stretching effected immediatelybefore, and a more preferable period is {fraction (1/10)} to ½. In thepresent invention, it is preferable that the stretching rate is 2000 to300000%/min in either the machine or transverse direction. A morepreferable range is 5000 to 200000%/min, and a further more preferablerange is 10000 to 100000%/min. It is preferable that the final filmforming speed in the machine direction is 200 m/min or more. Morepreferable is 300 m/min or more, and further more preferable is 400m/min or more. The stretching halting period in the above (1) can alsobe changed in reference to the stretching rate at that moment.

[0065] In the present invention, the stretching temperature forstretching the polyester film at a small ratio is not especiallylimited. When a cast film is drawn at a small ratio, it is preferable tokeep the temperature at (the glass transition temperature of thepolyester (Tg)+10)° C. to (Tg+120)° C. A more preferable range is(Tg+20)° C. to (Tg+80)° C. If the stretching temperature is lower than(Tg+10)° C., the orientation by stretching occurs too much, and it isdifficult to stretch to a high ratio.

[0066] On the other hand, if the stretching temperature exceeds(Tg+120)° C., it is difficult to make the polymer chains orientedslightly, which is necessary for easing the structure, and furthermorein the stretching step, the oligomer is scattered heavily. In thepresent invention it is preferable to stretch at a small ratio at whichthe yield point in the stress-strain curve of each stretchingtemperature is going to be reached. The reason is that under thiscondition, since the stretching tension corresponds to the strain at 1:1relation, the uniformity in the thickness of the film is littleadversely affected by stretching, and therefore that a high qualitypolyester film can be easily obtained. In the heat treatment effected at(Tg+120)° C. to less than the melting point for immobilizing thestructure of the film, the small-ratio stretching of the presentinvention is effective. In this case, it is preferable that the areastretching ratio is 1.5 times or less. More preferable is 1.2 times orless. If the small-ratio stretching is repeated in this ratio range, themechanical properties of the film are likely to be improved.

[0067] In the present invention, the small-ratio stretching can berepeated at any step while a cast film made of a resin mainly composedof a polyester is stretched and heat-treated to obtain a biaxiallyoriented polyester film. However, it is preferable to repeat thesmall-ratio stretching continuously three times or more in any step tillthe degree of crystallinity of the cast film reaches 3% to less than 30%or in said heat treatment step. The cast film in this case refers to afilm obtained by supplying sufficiently dried raw pellets into anextruder, extruding from a T die onto a revolving metallic casting drumas a sheet, and cooling and solidifying it or a film obtained bysupplying non-dried pellets into a vented extruder and processingsimilarly.

[0068] It is preferable to repeat the small-ratio stretchingcontinuously in the initial stretching step before the cast film easedin volume becomes high in the degree of crystallinity. When the degreeof crystallinity is less than 3%, even if the small-ratio stretching isrepeated continuously three times or more, it is difficult to eliminatethe mechanical strain caused in the subsequent simultaneous biaxialstretching, and it tends to be difficult to enhance the stretchingratio. In addition, the film is likely to decline in Young's modulus andto remarkably large in heat shrinkage. It is more preferable that thedegree of crystallinity of the film obtained by continuously repeatingthe small-ratio stretching of the present invention on the cast film is5% to less than 25%. A further more preferable range is 10% to less than20%. For the film of more than 30% in the degree of crystallinity, thesmall-ratio stretching can be repeated, but one time of stretching at ahigh ratio can also be effected. Especially when the polymer is liableto be crystallized due to the influence of additives, etc., it may bepreferable to stretch at a high ratio at a time than repeating thesmall-ratio stretching, for obtaining a film excellent in filmproperties and quality. Furthermore, a film of more than 30% in thedegree of crystallinity is likely to be eased in volume by thesmall-ratio stretching and is crystallized before stretching at a highratio, and it tends to be difficult to enhance the Young's modulus. Inthis case, any special measure such as stretching at a high ratio at atime must be taken.

[0069] The production method (II) is described below. This method mustcomprise effecting a series of operation to stretch and subsequentlyrelax a film, twice to less than 10000 times, and the total areastretching ratio must be 25 to 150 times.

[0070] In this case, “relaxation” refers to the operation to ease stressby carrying a film while holding both the edges of the film by clips,for relaxing the film in either the machine direction or the transversedirection. Furthermore, in the present invention, the film can also bestretched in either the machine direction or the transverse direction,while being relaxed in the other direction.

[0071] In the present invention, when stretching and relaxation areeffected simultaneously, the operation effected at an area stretchingratio of 1 or more is called “stretching”, and a case of less than 1,“relaxation”.

[0072] The “area stretching ratio” is the product obtained bymultiplying the dimensional change rate in the machine direction by thatin the transverse direction, and the “dimensional change rate” is therate of the length after stretching or relaxing to the original length.When the dimensional change rate is 1 or more, the value expresses astretching ratio, and when less than 1, the difference between thedimensional change rate (%) and 100 is the relaxation rate (%). In theprior arts, relaxation treatment is mainly effected after completion offilm stretching or in the cooling step after completion of stretchingand heat treatment. However, in the present invention, it is preferablethat a cast film obtained by melt-extruding into a sheet and casting itis stretched to be oriented, and relaxed at any stage before theintended final stretching ratio is reached.

[0073] As a stretcher capable of freely changing the directions of suchstretching and relaxation, stretching ratio and relaxation rate, it ispreferable to use a linear motor driven simultaneously biaxiallystretching tenter oven in the present invention. The features of alinear motor driven simultaneously biaxially stretching tenter oven are,as described before, that the film forming speed and the film width canbe increased to not lower or not smaller than those achieved in theconventional sequential biaxial stretching, that stretching at a highratio can be effected, and that the film deformation pattern can befreely changed in the steps of stretching, heat treatment andrelaxation. In the present invention, it is especially preferable forobtaining a polyester film with high film properties and quality at alow cost, to form the film using the linear motor driven simultaneouslybiaxially stretching tenter oven and by combining stretching andrelaxation.

[0074] In the present invention, the stretching stage when relaxation isto be effected before the cast film is stretched to a high ratio is notespecially limited, but the operation to stretch and subsequently relaxis effected twice to less than 10000 times. A more preferable range is 3to less than 1000 times, and a further more preferable range is 5 toless than 100 times. If the operation is effected only once, theintended effects of the present invention are small since the frequencyof relaxation is too small, and effecting the operation 10000 times ormore is unpreferable since practical difficulty is often involved. Thestretching and the relaxation can be effected in the machine directionand the transverse direction simultaneously, or in either directiononly. Furthermore, the series of operation repeated twice or more in thepresent invention includes not only the simply alternate operation ofstretching and relaxation, but also another series of operation with atleast one time of stretching or relaxation inserted between stretchingand relaxation, for example,

[0075] “-stretching-relaxation-relaxation-stretching” or

[0076] “-relaxation-stretching-stretching-relaxation-”.

[0077] In the production method (II), the area stretching ratio by onetime of stretching and the relaxation rate by one time of relaxation arenot especially limited. However, it is preferable that the areastretching ratio by one time of stretching is 1.005 to 10 times, andthat the relaxation rate is 0.1 to 80% to the respective lengths in themachine and transverse directions immediately before relaxation. A morepreferable area stretching ratio range by one time of stretching is 1.05to 5 times, and a further more preferable range is 1.1 to 3 times. Ifthe are a stretching ratio by one time of stretching exceeds 10 times,it is difficult to obtain the intended effects of the present invention,and film breaking may occur frequently. An area stretching ratio of lessthan 1.005 times cannot be a practical requirement, and it is oftendifficult to set the apparatus to achieve the ratio So, a preferablerange is 1.005 to 10 times. A more preferable relaxation rate range is0.5 to 60%, and a further more preferable range is 1 to 40%. If therelaxation rate exceeds 80%, the intended effects of the presentinvention by stretching become small and the film flatness andproductivity may be worsened. If the relaxation rate is less than 0.1%,it is often difficult to set the apparatus to achieve the ratio. So, apreferable range is 0.1 to 80%.

[0078] If the small-ratio stretching as in the production method (I) orthe series of operation consisting of stretching and relaxing as in theproduction method (II) is continuously repeated, the following effectscan be preferably obtained probably because the polyester chains in thefilm are disentangled.

[0079] (1) The easing of structure and volume is accelerated, and itbecomes easier to obtain a film with a high Young's modulus and smallheat shrinkage.

[0080] (2) The total area stretching ratio can be increased to improvefilm productivity and to allow cost reduction.

[0081] The dimension change rates of the small-ratio stretching(production method (I)) or stretching and relaxation (production method(II)) executed by a plurality of times can be equal or different everytime, and the respective stretching ratios and relaxation rates in themachine direction and the transverse direction can also be properlyselected to achieve the desired film properties. Furthermore, asdescribed before, the small-ratio stretching can also be effected ineither the machine direction or the transverse direction only. It isdesirable that the total ratio of the stretching ratio in the machinedirection to that in the transverse direction is 0.9 to 1.1 for makingthe film isotropic for application as a floppy disc, or 0.7 to 1.0 forapplication as a video tape, etc. used in a magnetic recorder with themagnetic head revolved helically, or 1.0 to 1.3 for application as adata tape used in a magnetic recorder with the magnetic head revolvedlinearly.

[0082] It is preferable that the sum of the Young's modulus (YMD) in themachine direction (MD direction) and the Young's modulus (YTD) in thetransverse direction (TD direction) of the film of the presentinvention, i.e., the total Young's modulus is in a range of 8 to 30 GPa,though depending on the polymer used. If the total Young's modulus isless than 8 GPa, the film is poor in practical applicability. If morethan 30 GPa, great difficulty may be involved, and the film breaking mayoccur frequently. A more preferable Young's modulus range is 10 to 25GPa, and an especially preferable range is 12 to 22 GPa. The balancebetween the Young's modulus in the machine direction and that in thetransverse direction can be controlled by properly changing therespective total ratios in the machine and transverse directions.

[0083] As for the heat shrinkage of the film obtained in the presentinvention, it is preferable that the sum of the heat shrinkagepercentage in the machine direction and that in the transverse directionat 100° C. for 30 minutes is 2% or less. A more preferable heatshrinkage sum range is 1% or less, and a further more preferable rangeis 0.5% or less. If the sum of heat shrinkage percentages is larger than2%, wrinkling and poor flatness, etc. are likely to occur for example,in the polyester processing process, or in the magnetic layer coatingstep, calendering step, etc. in the production of magnetic recordingmedia. So, it is preferable that the sum of heat shrinkage percentagesis 2% or less. According to the production method disclosed in thepresent invention, it is easier to enhance the Young's modulus in themachine and transverse directions without increasing the heat shrinkage.That is, it is easier to obtain a polyester film of 8 to 30 GPa in thesum of the Young's modulus in the machine direction and that in thetransverse direction and 2% or less in the sum of heat shrinkagepercentages at 100° C. for 30 minutes.

[0084] According to the production method of the present invention, theeasing of the polyester structure is likely to take place, and thebiaxially oriented and heat-treated film is likely to be higher in thedegree of crystallinity. As described before, the degree ofcrystallinity of a film is 30 to 90% in the present invention, thoughdepending on the polymer used, stretching ratio, heat treatmenttemperature, etc. Usually it is not easy to obtain a film of 50% or morein the degree of crystallinity by any industrially available productionmethod, but such a film can be relatively easily obtained according tothe production method of the present invention.

[0085] According to the production method of the present invention,since the film is likely to be higher in the degree of crystallinity,heat treatment at a temperature of 200° C. or higher is not necessarilyrequired. If the temperature of heat treatment is lowered, thecontamination in the tenter oven by scattering of the oligomer and theamount of the oligomer on the film surface decrease advantageously fordecreasing surface defects. A preferable range in the degree ofcrystallinity for obtaining a high quality polyester film with a highYoung's modulus and small heat shrinkage is 40 to 80%, and a furthermore preferable range is 45 to 70%. If the degree of crystallinity isless than 30%, the immobilization of structure is often insufficient,and the heat shrinkage of the film becomes large unpreferably. If thedegree of crystallinity exceeds 90%, film breaking occurs frequently, tolower processability in various film applications.

[0086] The polyester film of the present invention can contain inorganicparticles and organic particles and other various additives such asantioxidant, antistatic agent and crystal nucleating agent. Thecompounds which can be used as the inorganic particles include, thoughnot limited to, oxides such as silicon oxide, aluminum oxide, magnesiumoxide and titanium oxide, compound oxides such as kaolin, talc andmontmorillonite, carbonates such as calcium carbonate and bariumcarbonate, sulfates such as calcium sulfate and barium sulfate,titanates such as barium titanate and potassium titanate, phosphatessuch as calcium tertiary phosphate, calcium secondary phosphate andcalcium primary phosphate, etc. Depending on the purpose, two or more ofthese compounds can also be used in combination.

[0087] The organic particles which can be used here include, though notlimited to, vinyl based particles such as polystyrene particles orcrosslinked polystyrene particles, styrene-acrylic particles or acryliccrosslinked particles, styrene-methacrylic particles or methacryliccrosslinked particles, benzoguanamine-formaldehyde particles, siliconeparticles, polytetrafluoroethylene particles, etc. Any organic polymericfine particles, at least a component of which is insoluble in thepolyester, can be used. It is also preferable that the organic particlesare spherical and have a uniform particle size distribution in view oflubricity and uniform formation of projections on the film surface. Theparticle size, amount, form, etc. of the particles can be selected tosuit the application and purpose concerned. Usually it is preferablethat the average particle size is 0.05 μm to 3 μm, and that the particlecontent is 0.01 wt % to 10 wt %.

[0088] The polyester film of the present invention can be a single-layerfilm, but can also be a laminate film consisting of two or more layers,in which a layer of another polymer such as a polyester, polyolefin,polyamide, polyvinylidene chloride or acrylic polymer is laminateddirectly or via an adhesive layer, etc. Especially a laminate film witha polyester layer laminated as a surface layer is useful as a base filmfor a magnetic recording medium in which surface properties areimportant, since the surface roughness of the film face as a magneticrecording face can be designed to be different from that on the otherside to suit the application concerned.

[0089] The entire thickness of the film in the present invention can bedecided properly to suit the application and purpose of the filmconcerned.

[0090] Usually for magnetic material application, it is preferable thatthe thickness is 1 μm to 20 μm. Above all, for a coating type magneticrecording medium for digital video, the preferable thickness range is 2μm to 8 μm, and for a vapor deposition type magnetic recording mediumfor digital video, the preferable thickness range is 3 μm to 9 μm.Furthermore, among industrial material applications, the preferablethickness range for a heat transfer ribbon is 1 μm to 6 μm, and that fora capacitor is 0.5 μm to 15 μm. The preferable thickness range forthermal mimeographic stencil paper is 0.5 μm to 5 μm.

[0091] Examples of the polyester film production method of the presentinvention are described below, but the present invention is not ofcourse limited thereto or thereby. As the polyester, polyethyleneterephthalate is used in the following examples, and productionconditions depend on the polyester used.

[0092] As the polyester, pellets of polyethylene terephathalate with aninherent viscosity of 0.65 are heated to 180° C. in vacuum, to be driedin vacuum for 3 hours or more, and are supplied into an extruder heatedto a temperature of 270 to 300° C., being extruded from a T die as asheet. To remove foreign matters and deteriorated polymer, it ispreferable to use various filters, for example, filters of suchmaterials as sintered metal, porous ceramic material, sand and wiregauze. Furthermore, as required, for better quantitative supply, a gearpump may also be used. The molten sheet is electrostatically broughtinto contact with a drum cooled to a surface temperature of 10 to 40°C., to be cooled and solidified, for obtaining a substantially amorphouscast film. In the case of a laminate film, two or more extruders and amanifold or laminating block are used to extrude a sheet with a moltenpolyester laminated. In this case, it is preferable that the ratio ofthe thickness of the cast film at the edges to the thickness of the filmat the center is 1 to 10. A more preferable range is 1 to less than 5,and the most preferable range is 1 to less than 3. If the thicknessratio is less than 1 or more than 10, film breaking or clipping-offoccurs frequently unpreferably. Then, the cast film is introduced into alinear motor driven simultaneously biaxially stretching tenter oven byholding both the edges of the film by clips, heated to 90˜150° C. in apreheating zone, and stretched at a small ratio of 1.0005 to 3 times inarea stretching ratio at least three times or more continuously. Asanother method, the film is stretched to achieve an area stretchingratio of 1.005 to 10 times and relaxed at a relaxation rate of 0.1 to80%, and this series of operation is effected continuously at leasttwice or more. In either method, it is preferable that the temperatureof the clips used for holding the edges of the film is set in a range of80 to 160° C. It is preferable that the stretching temperature in thestretching process is kept in a range of 90 to 150° C. However, the filmcan be once cooled, and stretched while inhibiting the crystallizationof the film. Furthermore, when the polymer has a high molecular weightor is unlikely to be crystallized, it is preferable to raise thestretching temperature up to 200° C. It is also preferable that in thelatter half of the stretching process, i.e., when a film with a planeorientation factor of 0.15 or more is stretched,.the stretchingtemperature is gradually raised in 2 or more steps, during stretching.The stretching of the film by a simultaneously biaxially stretchingtenter oven is effected as described above, to achieve a total areastretching ratio of 25 to 150 times. Then, the biaxially orientedpolyester film is heat-treated in a range of 180° C. to less than themelting point, and relaxed preferably in a temperature range of 100 to220° C. in the machine and transverse directions, preferably in a rangeof 1 to 6% in each direction in the cooling step from the heatsettemperature, to make the film flat and stable dimensionally. Therelaxation treatment can be effected in one or more steps, and thetemperature distribution may also be changed. In this case, repeatingthe small-ratio stretching in the heat treatment step is also preferablefor increasing the crystal size for enhancing the Young's modulus of thefilm. Then, the film is cooled to room temperature while being relaxedin the machine and transverse directions as required, and wound toobtain the intended polyester film.

[0093] In the present invention, to let the film have desirable surfaceproperties such as adhesiveness, lubricity, releasability andelectrification control, it is also preferable to coat the surface ofthe polyester film with any appropriate material, before or afterstretching of the film by a simultaneously biaxially stretching tenteroven.

[0094] [Methods for Evaluating Physical Properties]

[0095] (1) Inherent viscosity [η]

[0096] The solution viscosity measured in orthochlorophenol at 25° C. isused to calculate the value from the following formula:

ηsp/C=[η]+K[η]2·C

[0097] where ηsp=(solution viscosity/solvent viscosity)−1; C is theweight of the molten polymer per 100 ml of the solvent (g/100 ml,usually 1.2); and K is Huggins' constant (0.343). The solution viscosityand the solvent viscosity were measured using an Ostwald viscometer.Inherent viscosity determined is expressed in unit of [dl/g].

[0098] (2) Glass transition temperature Tg, and melting temperature Tm

[0099] As the differential scanning calorimeter, “Robot DSC-RDC220”produced by Seiko Denshi Kogyo K. K. was used, and as the data analyzer,“Disc Session” SSC/5200 produced by the same manufacturer was used. Asample was taken by about 5 mg, and from the thermal curve obtained byheating from room temperature to 300° C. at a heating rate of 20°C./min, Tg and Tm were obtained.

[0100] (3) Young's modulus

[0101] Measured according to the method specified in ASTM D 882. Filmstrength-elongation automatic measuring instrument, “TensilonAMF/RTA-100” produced by Orienteck K.K. was used to pull a 10 mm widesample film at a gauge length of 100 mm at a tensile speed of 200mm/min. From the gradient of the tangential line at the rise of theobtained stress-strain curve, Young's modulus was obtained. Themeasurement was effected in an atmosphere of 23° C. and 65% RH.

[0102] (4) Heat shrinkage

[0103] Measured according to the method specified in JIS C 2318. Twolines were drawn on a 10 mm wide film to identify a measuring length ofabout 200 mm, and the distance between the two lines was accuratelymeasured as L0. The sample was allowed to stand in a 100° C. oven for 30minutes without any load, and the distance between the two lines wasmeasured as L1. The heat shrinkage was obtained from the followingformula:

Heat shrinkage (%)={(L0−L1)/L0}×100

[0104] (5) Degree of crystallinity

[0105] Obtained from the density gradient, according to the methodspecified in JISK7112. Density gradient tubes of sodium bromide aqueoussolutions were prepared, to measure the density of the film at 25° C.From the density d, the degree of crystallinity was obtained using thefollowing formula:

Degree of crystallinity (%)=((d−da)/(dc−da))×100

[0106] where da is the amorphous density and dc is the perfectlycrystalline density. In the case of polyethylene terephthalate, da=1.335and dc=1.455 g/cm³ according to literature.

[0107] (6) Plane orientation factor

[0108] Refractive indexes were measured according to the methodspecified in JIS K 7105. As the light source, a sodium lamp was used,and the refractive indexes of the film (machine direction: Na,transverse direction: Nb, normal direction: Nc) were obtained by anAbbe's refractometer (produced by Atago), and the plane orientationfactor F was calculated from the following formula. As the mount liquid,methylene iodide was used, and measurement was effected in an atmosphereof 23° C. and 65% RH.

F=[(Na+Nb)/2]−Nc

[0109] (7) Breaking frequency

[0110] Polyethylene terephthalate dried in vacuum was electrostaticallybrought into contact with a casting drum from a T die, to be cooled andsolidified, for obtaining a cast film, and film breaking in the filmformation by a linear motor driven simultaneously biaxially stretchingtenter oven was observed and evaluated according to the followingcriterion:

[0111] ⊚: Film breaking did not occur at all.

[0112] ◯: Film breaking occurred very rarely.

[0113] Δ: Film breaking occurred sometimes.

[0114] X: Film breaking occurred frequently.

[0115] (8) Thickness fluctuation of film in the longitudinal directionThe thickness of a 30 mm wide and 10 m long sample film take in themachine direction was continuously measured using film thickness tester,“KG601A” and electronic micrometer, “K306C” respectively produced byAnritsu Corp. The film was fed at a rate of 3 m/min. From the maximumthickness Tmax (μm) and the minimum thickness Tmin (μm) of the 10 mlength,

R=Tmax−Tmin

[0116] was obtained, and from the average thickness Tave (μm) of the 10m length, the thickness fluctuation was obtained from the followingformula:

Thickness fluctuation (%)=(R/Tave)×100

[0117] (9) Creep compliance

[0118] A 4 mm wide film sample was set in TMA (TM-3000) and heat controlsection TA-1500 produced by Shinku Riko K. K., at a gauge length of 15mm.

[0119] A load of 28 MPa was applied to the film at 50° C. and 65% RH,and kept for 30 minutes, and the elongation of the film in this case wasmeasured. The elongation of the film (in %,

L) was obtained by personal computer, PC-9801 produced by NEC Corp. viaAD converter ADX-98E produced by Kanops Denshi K. K., and the creepcompliance was calculated from the following formula:

Creep compliance (GPa⁻¹)=(

L/100)/0.028

[0120] (10) High speed abrasion resistance

[0121] A tape obtained by slitting a film, to have a width of ½″ wasdriven to run on a guide pin (surface roughness: 100 nm as Ra) using atape running tester (running speed 250 m/min, running frequency 1 pass,wrap angle 60°, running tension 90 g). After completion of film running,the guide pin was visually observed and evaluated according to thefollowing criterion:

[0122] ◯: Deposition of white powder was not observed.

[0123] Δ: Some deposition of white powder was observed.

[0124] X: Much deposition of white powder was observed.

[0125] A tape evaluated as o is desirable, but a tape evaluated as Δ isstill practically usable.

[0126] (11) Electromagnetic conversion properties of magnetic tape (C/N)

[0127] A polyester film of the present invention was coated on thesurface with a magnetic coating material composed as follows and anon-magnetic coating material composed as follows using an extrusioncoater (the magnetic coating material was applied as an upper layer witha thickness of 0.1 μm, and the non-magnetic coating material was appliedas a lower layer with the thickness changed), magnetically oriented anddried. Then, the polyester film was coated on the other side with a backcoat layer composed as follows, and the coated film was calendered by asmall test calender (steel/steel rolls, 5 stages) at a temperature of85° C. at a linear pressure of 200 kg/cm, and cured at 60° C. for 48hours. The film was slit into an 8 mm wide tape, to prepare a pan cake.From the pan cake, a 200 m long sample was taken and installed in acassette, to make a cassette tape.

[0128] The tape was setin a marketed VTR for Hi 8 (EV-BS 3000 producedby SONY), to measure the C/N (carrier to noise ratio) at 7 MHz+1 MHz.The C/N was compared with that of a marketed video tape for Hi 8(120-min MP produced by SONY) and evaluated according to the followingcriterion:

[0129] ◯: +3 dB or more

[0130] Δ: +1 dB to Less than +3 dB

[0131] X: Less than +1 dB

[0132] A tape evaluated as o is desirable, but even a tape evaluated asΔ is practically usable. (Composition of magnetic coating material)Ferromagnetic metallic powder 100 parts by weight Sodium sulfonatemodified vinyl chloride 10 parts by weight copolymer Sodium sulfonatemodified polyurethane 10 parts by weight Polyisocyanate 5 parts byweight Stearic acid 1.5 parts by weight Oleic acid 1 part by weightCarbon black 1 part by weight Alumina 10 parts by weight Methyl ethylketone 75 parts by weight Cyclohexanone 75 parts by weight Toluene 75parts by weight

[0133] (Composition of non-magnetic coating material for lower layer)Titanium oxide 100 parts by weight Carbon black 10 parts by weightSodium sulfonate modified vinyl chloride 10 parts by weight copolymerSodium sulfonate modified polyurethane 10 parts by weight Methyl ethylketone 30 parts by weight Methyl isobutyl ketone 30 parts by weightToluene 30 parts by weight

[0134] (Composition of back coat) Carbon black (average particle size 20nm) 95 parts by weight Carbon black (average particle size 280 nm) 10parts by weight α alumina 0.1 part by weight Zinc oxide 0.3 part byweight Sodium sulfonate modified polyurethane 20 parts by weight Sodiumsulfonate modified vinyl chloride 30 parts by weight copolymerCyclohexanone 200 parts by weight Methyl ethyl ketone 300 parts byweight Toluene 100 parts by weight

[0135] (12) Running durability and storage stability of magnetic tape

[0136] A polyester film of the present invention was coated on thesurface with a magnetic coating material composed as follows to have athickness of 20 μm, magnetically oriented and dried. Then, it was coatedon the other side with a back coat layer composed as follows, calenderedand cured at 60° C. for 48 hours. The film was slit into a ½″ widemagnetic tape, and a 670 m long sample was taken from it and installedin a cassette, to make a cassette tape. (Composition of magnetic coatingmaterial) Ferromagnetic metallic powder 100 parts by weight Modifiedvinyl chloride copolymer 10 parts by weight Modified polyurethane 10parts by weight Polyisocyanate 5 parts by weight Stearic acid 1.5 partsby weight Oleic acid 1 part by weight Carbon black 1 part by weightAlumina 10 parts by weight Methyl ethyl ketone 75 parts by weightCyclohexanone 75 parts by weight Toluene 75 parts by weight

[0137] (Composition of back coat) Carbon black (average particle size 20nm) 95 parts by weight Carbon black (average particle size 10 parts byweight 280 nm) α alumina 0.1 part by weight Modified polyurethane 20parts by weight Modified vinyl chloride copolymer 30 parts by weightCyclohexanone 200 parts by weight Methyl ethyl ketone 300 parts byweight Toluene 100 parts by weight

[0138] The prepared cassette tape was driven to run for 100 hours usingMagstar 3590 Model B1A Tape Drive produced by IBM, and the tape runningdurability was evaluated according to the following criterion. A tapeevaluated as o is acceptable.

[0139] ◯: The tape was not elongated or folded at its edges and did notshow any abrasion mark.

[0140] Δ: The tape was not elongated or folded at its edges, but showedsome abrasion marks.

[0141] X: The tape was partially elongated and damaged at the edges,showing abrasion marks.

[0142] The cassette tape prepared as above had data written usingMagstar 3590 Model B1A Tape Drive produced by IBM, and it was stored inan atmosphere of 40° C. and 80% RH for 100 hours. Then, the data werereproduced, to evaluate the storage stability of the tape according tothe following criterion. A tape evaluated as o is acceptable.

[0143] ◯: The data could be reproduced normally without any shift intrack.

[0144] Δ: The tape was normal in width, but data could not be partiallyreproduced.

[0145] X: The tape changed in width, and some data could not bereproduced.

[0146] (13) Tracking resistance of floppy disc

[0147] A. Temperature affected tracking shift test

[0148] As the tracking shift test, the following method was used. Thinmetallic layers were formed as magnetic recording layers on both sidesof a substrate film by sputtering, and the laminate was punched into afloppy disc with thin metallic layers. A ring head was used for magneticrecording at 15° C. and 60% RH, and the maximum output and the outputenvelop of the magnetic sheet were measured. Then, the floppy disc waskept at 40° C. and 60% RH, and the maximum output and output envelopwere measured. The output envelop measured at 15° C. and 60% RH and thatmeasured at 40° C. and 60% RH were compared, to examine the trackingstate. If the difference is smaller, the tracking resistance is better.The tracking resistance was evaluated according to the followingcriterion:

[0149] X: The difference of output envelops was more than 3 dB.

[0150] ◯: The difference of output envelops was 3 dB or less.

[0151] B. Humidity affected tracking shift test

[0152] The floppy disc prepared as described above had data recorded inan atmosphere of 25° C. and 20% relative humidity, and furthermore keptin an atmosphere of 25° C. and 70% relative humidity, to compare theoutput envelops under those conditions, for examining the trackingstate. As in the previous item, the tacking resistance was evaluatedaccording to the following criterion:

[0153] X: The difference of output envelops was more than 3 dB.

[0154] ◯: The difference of output envelops was 3 dB or less.

[0155] (14) Scratch resistance of floppy disc

[0156] The same track magnetically recorded on a floppy disc obtained asdescribed for (13) was scanned more than 100,000 times at a relativerunning speed of 6 m/sec, to examine the output envelop and thescratches formed on the surface of the magnetic layer. The scratchresistance was evaluated according to the following criterion:

[0157] X: Scratches were formed on the surface and the output envelopwas unstable.

[0158] ◯: Scratches were not formed on the surface and the outputenvelop was stable.

[0159] (15) Printability on heat transfer ribbon

[0160] A polyester film for heat transfer ribbon of the presentinvention was coated on one side with a fusion preventive layer, andcoated on the other side with a heat transfer ink composed as follows tohave a thickness of 3.5 μm by a hot melt coater, for producing a heattransfer ribbon. (Composition of heat transfer ink) Carnauba wax 60.6 wt% Microcrystalline wax 18.2 wt % Vinyl acetate ethylene copolymer  0.1wt % Carbon black 21.1 wt %

[0161] On the prepared heat transfer ribbon, black solid was printed bya bar code printer (BC-8) produced by Oaks, to evaluate printability. Aribbon evaluated as o is acceptable.

[0162] ◯: Clearly printed.

[0163] Δ: Printing shifted in pitch.

[0164] X: The ribbon was wrinkled, to disorder printing.

[0165] XX: During hot melt coating, the film was wrinkled, and could notbe coated with the heat transfer ink uniformly.

[0166] (16) Evaluation of properties for capacitor

[0167] A. Insulation resistance

[0168] A polyester film of the present invention had aluminumvapor-deposited in vacuum to have a surface resistance value of 2 Ω/sq.In this case, the aluminum was deposited in stripes with longitudinalmargins (57 mm wide aluminum deposited portions and 3 mm wide marginswere formed alternately). Then, the film was slit at the centers of therespective aluminum deposited portions and at the centers of therespective margins by blades, to obtain 30 mm wide tapes with a 1.5 mmmargin on the left or right respectively. They were wound into reels. Analuminum deposited film with a margin on the left and that with a marginon the right were overlaid as one pair, and wound at a length to give acapacity of 1.5 μF. The wound product was formed by pressing at 120° C.at a pressure of 20 kg/cm² for 10 minutes. On both the ends, metallikonwas thermally sprayed as electrodes, and lead wires were attached, tomake a capacitor sample. In this way, one thousand 1.5 μF capacitorsamples were prepared and their insulation resistances were measured asone-minute values at an applied voltage of 500 V by a super-insulationresistance tester 4329A produced by YHP in an atmosphere of 23° C. and65% RH. Capacitor samples of less than 500 MΩ in insulation resistancewere judged to be defective. The lot was evaluated according to thefollowing criterion. A lot evaluated as ⊚,

[0169] ◯ or Δ is acceptable in the present invention.

[0170] ⊚: The number of defective samples was less than 10.

[0171] ◯: The number of defective samples was 10 to less than 20.

[0172] Δ: The number of defective samples was 20 to less than 50.

[0173] X: The number of defective samples was 50 or more.

[0174] B. Dielectric breakdown voltage

[0175] The dielectric breakdown voltage was evaluated as described belowaccording to the method stated in JIS C 2318, but using a non-metallizedfilm as a specimen.

[0176] An about 2 mm thick rubber sheet with a Shore hardness of about60° was laid on a metallic flat plate with a proper size, and tenoverlaid about 6 μm thick aluminum foils were placed on it as a bottomelectrode while an about 50 g brass cylinder with a diameter of 8 mm,with about 1 mm roundness around it and with a smooth flaw less bottomface was placed as a to pelectrode. The specimen had been allowed tostand in an atmosphere of 20±5° C. and 65±5% relative humidity for morethan 48 hours beforehand. The specimen was held between the topelectrode and the bottom electrode, and a DC voltage was applied betweenboth the electrodes from a DC power supply in an atmosphere of 20±5° C.and 65±5% relative humidity and raised from 0 V at a rate of 100 V persecond till dielectric breakdown occurred. Fifty specimens were tested,and the respective dielectric breakdown voltages were divided by thethickness of the specimen. The quotients were averaged, and when thevalue was 400 V/μm or more, the lot was acceptable (◯).

[0177] (17) Image property of thermal mimeographic stencil paper

[0178] To a polyester film of the present invention, a nonwoven fabricobtained by the following method was bonded using a vinyl acetate basedadhesive. The film was coated with a silicone based releasing agent onthe side opposite to the nonwoven fabric side, to obtain thermalmimeographic stencil paper. The stencil paper was supplied into“RISOGRAPH” GR375 produced by Riso Kagaku Kogyo K. K., and a black solidoriginal was used for making printing paper. The printing paper was usedfor printing 20 sheets, and the voids and shade fluctuation of theprinted image of the 20th sheet were visually observed and evaluatedaccording to the following criteria:

[0179] (Voids)

[0180] ◯: No void was observed at all.

[0181] Δ: Some voids were observed.

[0182] X: Voids were remarkably observed.

[0183] (Shade Fluctuation)

[0184] ◯: No shade fluctuation was observed at all.

[0185] Δ: Some shade fluctuation was observed.

[0186] X: Shade fluctuation was remarkably observed.

[0187] The printing paper evaluated as ◯ or Δ is practically usable.

[0188] [Production of Main Fibers]

[0189] Chips of polyethylene terephthalate were molten at 290° C., andthe molten polymer was discharged from a die with 900 holes at 285° C.The fibers were wound at a speed of 1000 m/min.

[0190] The non-stretched fibers were stretched to 3.8 times in 80° C.water, heatset under tension at 200° C., heatset with relaxation at 125°C., and cut at 5 mm, to obtain main fibers A with an average fiberdiameter of 5 μm and a birefringence of 0.20.

[0191] [Production of Non-stretched Fibers]

[0192] On the other hand, polyethylene terephthalate chips were moltenat 290° C., and the molten polymer was discharged from a die with 900holes at 285° C., and cut at 5 mm, to obtain non-stretched fibers a withan average fiber diameter of 8 μm with a birefringence of 0.05.

[0193] [Paper Making]

[0194] The main fibers A and the non-stretched fibers a weresufficiently mixed and dispersed at a ratio by weight of 80:20 in apulper, and the mixture was processed by a cylinder paper machine at aspeed of 10 m/min, and heated and dried by a Yankee drier (surfacetemperature 130° C.). The paper had an a real unit weight of 8 g/m².Then, it was pressed by a calender of metallic/elastic rolls at ametallic roll surface temperature of 210° C. at a linear pressure of 15kg/cm, to obtain a 25 μm thick nonwoven fabric.

EXAMPLES

[0195] The present invention is described below based on examples andcomparative examples.

Example 1

[0196] Pellets of polyethylene terephthalate (inherent viscosity 0.65,glass transition temperature 75° C., melting point 255° C., containing0.1 wt % of spherical crosslinked polystyrene particles with an averageparticle size of 0.3 μm) are dried in vacuum at 180° C. for 3 hours,supplied into an extruder heated to 280° C., and melt-extruded from a Tdie as a sheet. The sheet is electrostatically brought into contact witha cooling drum with a surface temperature of 25° C., to be cooled andsolidified, for obtaining a cast film. The cast film is held at both theedges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, and heated to a film temperature of100° C., and simultaneous biaxial small-ratio stretching at an areastretching ratio of 1.082 times (1.04 times in the machine direction,and 1.04 times in the transverse direction) is effected 50 timescontinuously. The stretching halt period between the respectivelysuccessive two times of small-ratio stretching is {fraction (1/10)} ofthe time taken for the small-ratio stretching effected immediatelybefore. Then, the film is heatset at 210° C., relaxed in a 120° C.cooling zone at a relaxation rate of 2% in the machine direction and 2%in the transverse direction, gradually cooled to room temperature, andwound. The film is controlled to have a thickness of 9 μm by adjustingthe amount of extrusion. The clip temperature at the time of stretchingis 100° C. The obtained film reached an area stretching ratio of about50 times, and is as high as 58% in the degree of crystallinity, being ahigh quality film less in thickness fluctuation with both a high Young'smodulus and small heat shrinkage.

[0197] At the time of film formation, film breaking little occurred, anda film with high film properties and quality could be very stablyobtained.

Examples 2 to 5 and Comparative Example 1

[0198] Biaxially oriented polyester films are obtained as described forExample 1, except that the ratio and repetition frequency of thesmall-ratio stretching, and total area stretching ratio are changed. InExample 2 and Comparative Example 1 where the small-ratio stretching wasrepeated 3 times and twice, simultaneous biaxial stretching is effectedby one step after completion of small-ratio stretching, to achieve atotal area stretching ratio of 25 times. When the repetition frequencyof small-ratio stretching is increased from 3 times, film breakingfrequency declined, and the total area stretching ratio tended to behigher. When the repetition frequency of small-ratio stretching isincreased for stretching at higher ratios, the film became higher in thedegree of crystallinity, and a high quality film with high rigidity,small heat shrinkage and less thickness fluctuation could be obtained.TABLE 1 Simultaneous biaxial small-ratio stretching conditions Young'sArea stretching ratio Total area modulus Heat shrinkage Degree ofThickness (ratio in machine direction × ratio Repetition stretching(GPa) (MD/TD) (%) cryscallinity Breaking fluctuation in transversedirection) frequency ratio (MD/TD) (100° C., 30 min) (%) frequency (%)Example 1 1.082 (1.04 × 1.04) 50 50.5 7.8/8.0 0.4/0.3 58 ⊚ 5 Example 22.250 (1.50 × 1.50) 3 25.0 5.5/5.4 0.9/0.8 46 ◯ 7 Example 3 1.440 (1.20× 1.20) 10 38.3 6.5/6.4 0.4/0.3 52 ◯ 5 Example 4 1.020 (1.01 × 1.01) 21065.3 8.2/8.1 0.3/0.2 62 ⊚ 4 Example 5 2.250 (1.50 × 1.50) 5 57.7 7.6/7.81.0/0.7 50 ◯ 6 Comparative 2.250 (1.50 × 1.50) 2 25.0 4.8/4.5 1.2/1.3 43Δ 10  Example 1

Comparative Examples 2 to 4

[0199] Biaxially oriented polyester films are obtained as described forExample 1 except that the films are stretched without small-ratiostretching. When a 100° C. film is stretched to 4.3 times in the machinedirection and subsequently to 4.3 times in the transverse direction by asimultaneously biaxially stretching tenter oven or simultaneouslybiaxially stretched to 4.3 times respectively in the machine andtransverse directions, the film obtained is low in Young's modulus,large in heat shrinkage and also large in thickness fluctuation(Comparative Examples 2 and 3). When a film is simultaneously biaxiallystretched to 4.0 times respectively in the machine and transversedirections and subsequently simultaneously stretched to 1.3 timesrespectively in the machine and transverse directions, film breakingoccurs frequently, and the heat shrinkage of the film becomes large(Comparative Example 4). TABLE 2 Stretching conditions Total areaYoung's modulus Heat shrinkage Degree of Thickness Ratio of 1st stepRatio of 2nd step stretching (GPa) (MD/TD) (%) crystallinity Breakingfluctuation (stretching mode) (stretching mode) ratio (MD/TD) (100° C.,30 min) (%) frequency (%) Comparative 4.3 × 4.3 No stretching 18.54.4/4.5 0.7/0.6 42 Δ 10 Example 2 (longitudinal and lateral sequential)Comparative 4.3 × 4.3 (simultaneous) No stretching 18.5 3.8/4.2 0.5/0.642 ◯ 11 Example 3 Comparative 4.0 × 4.0 (simultaneous) 1.3 × 1.3 27.06.2/6.1 1.4/1.5 44 X 9 Example 4 (simultaneous)

Examples 6 to 10

[0200] In these examples, the degree of crystallinity reached aftersmall ratio-stretching is changed. Biaxially oriented polyester filmsare obtained as described in Example 1 except that the ratio andrepetition frequency of small-ratio stretching are changed and that thetotal area stretching ratio is set at 50 times by one step ofsimultaneous biaxial stretching after continuously repeating thesmall-ratio stretching. The stretching ratios in the machine andtransverse directions by one time of small-ratio stretching are equal.When the degree of crystallinity of the film after small-ratiostretching is 2% or 34%, the Young's modulus is low, and the heatshrinkage large. TABLE 3 Simultaneous biaxial small-ratio stretchingconditions Young's modulus Heat shrinkage Degree of Area stretchingratio by Repetition Degree of crystallinity after (GPa) (MD/TD) (%)crystallinity small-ratio stretching frequency small-ratio stretching(%) (MD/TD) (100° C., 30 min) (%) Example 6 1.020 40 2 6.2/6.4 1.1/0.949 Example 7 1.082 30 4 7.4/7.5 0.6/0.7 55 Example 8 1.020 100 7 7.9/8.20.5/0.5 58 Example 9 1.020 150 15 8.3/8.4 0.4/0.5 60 Example 10 1.210 2034 7.3/7.4 0.9/0.8 53

Examples 11 to 13

[0201] Biaxially oriented polyester films are obtained as described forExample 4, except that temperature zones of 100° C., 140° C., 210° C.and 250° C. are established in the film flow direction in thesimultaneously biaxially stretching tenter oven, to change thetemperature condition of the small-ratio stretching. When thesmall-ratio stretching is effected in high temperature zones of 210° C.and 250° C., the film is raised in Young's modulus and lowered in heatshrinkage. TABLE 4 Repetition frequencies of simultaneous biaxialsmall-ratio Young's modulus Heat shrinkage Degree of stretching inrespective temperature zones Total area (GPa) (MD/TD) (%) crystallinity100° C. 140° C. 210° C. 250° C. stretching ratio (MD/TD) (100° C., 30min) (%) Example 11 100 50 50 10 65.3 8.8/8.6 0.3/0.3 64 Example 12 10010 50 50 65.3 9.0/9.2 0.2/0.2 66 Example 13 100 110 0 0 65.3 8.4/8.30.4/0.3 61

Example 14

[0202] A cast film obtained as described for Example 1 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, heated to a film temperature of 100°C., and stretched to 2.5 times respectively in the machine direction andthe transverse direction sequentially, and the small-ratio stretching toan area stretching ratio of 1.44 times (1.2 times in the machinedirection and 1.2 times in the transverse direction) is effected 6 timescontinuously. In this case, temperature zones of 150° C., 180° C. and210° C. are established in this order, and the small-ratio stretching iseffected twice respectively. The stretching halt period between therespectively successive two times of small-ratio stretching is {fraction(1/10)} of the time taken for the small-ratio stretching effectedimmediately before. Then, the film is heatset at 210° C., relaxed in a120° C. cooling zone at a relaxation rate of 2% in the machine directionand 2% in the transverse direction, gradually cooled to roomtemperature, and wound. The film thickness is controlled to have athickness of 10 μm by adjusting the amount of extrusion. The filmobtained had a high Young's modulus and low heat shrinkage.

Example 15

[0203] A cast film obtained as described in Example 1 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, and heated to a film temperature of115° C., and small-ratio stretching to 1.04 times in the machinedirection is effected 20 times continuously. The film is thensimultaneously biaxially stretched at 80° C. to 4 times in the machinedirection and 5 times in the transverse direction. The stretching haltperiod between the respectively successive two times of small-ratiostretching is {fraction (1/10)} of the time taken for the small-ratiostretching effected immediately before. Then, the film is heatset at210° C., relaxed in a 120° C. cooling zone at a relaxation rate of 2% inthe machine direction and 2% in the transverse direction, graduallycooled to room temperature, and wound. The film is controlled to have athickness of 10 μm by adjusting the amount of extrusion. The filmobtained had a high Young's modulus and low heat shrinkage.

Example 16

[0204] A cast film obtained as described for Example 1 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, and heated to a film temperature of100° C., and small-ratio stretching to an area stretching ratio of 1.082times (1.04 times in the machine direction and 1.04 times in thetransverse direction) and small-ratio stretching to 1.04 times in thetransverse direction only are effected respectively alternately 10 times(20 times of small-ratio stretching in total). Then, the film issimultaneously biaxially stretched to 4 times respectively in themachine and transverse directions. The stretching halt period betweenthe respectively successive two times of small-ratio stretching is{fraction (1/10)} of the time taken for the small-ratio stretchingeffected immediately before. Then, the film is heatset at 210° C.,relaxed in a 120° C. cooling zone at a relaxation rate of 2% in themachine direction and 2% in the transverse direction, gradually cooledto room temperature, and wound. The film is controlled to have athickness of 10 μm by adjusting the amount of extrusion. The filmobtained had a high Young's modulus and low heat shrinkage. TABLE 5Stretching conditions 1st step 2nd step Ratio in machine direction ×Ratio in machine direction × Young's ratio in ratio in Total areamodulus Heat shrinkage Degree of transverse direction, (stretchingtransverse direction, (stretching stretching (GPa) (MD/TD) (%)crystallinity mode) repetition frequency mode) repetition frequencyratio (MD/TD) (100° C., 30 min) (%) Example 14 2.5 × 2.5 1.2 × 1.2(simultaneous) 6 times 55.7 7.6/7.8 0.7/0.5 52 (longitudinal lateralsequential) 1 time Example 15 1.04 × 1.0 (longitudinal) 20 times 4 × 5(simultaneous) 1 time 43.8 6.5/6.7 0.6/0.5 55 Example 16 1.04 × 1.04(simultaneous) 10 4 × 4 (simultaneous) 1 time 52.2 7.7/7.8 0.5/0.4 56times 1.0 × 1.04 (lateral) 10 times

Example 17 and Comparative Example 5

[0205] Polyethylene terephthalate with an inherent viscosity of 1.0(glass transition temperature 74° C., melting point 255° C., containing0.1 wt % of spherical crosslinked polystyrene particles with an averageparticle size of 0.3 μm) is used as a raw polyester, to examine theeffects of simultaneous biaxial small-ratio stretching. Six point fivemicrometers thick biaxially oriented polyester films are obtained asdescribed for Example 1, except that the temperature of the stretchingzone is 115° C., that the temperature of the heat treatment zone is 210°C. and that the stretching pattern is changed. When small-ratiostretching is effected, small-ratio stretching to achieve an areastretching ratio of 1.082 times (1.04 times in the machine direction and1.04 times in the transverse direction) is repeated 50 timescontinuously. The stretching halt period between the respectivelysuccessive two times of small-ratio stretching was {fraction (1/10)} ofthe time taken for the small-ratio stretching effected immediatelybefore. When the small-ratio stretching is not effected, simultaneousbiaxial stretching at equal ratios in the machine and transversedirections is effected in one step. Unlike the case of ComparativeExample 5, in Example 17 where the small-ratio stretching is effected,the total area stretching ratio is high, and the film obtained had ahigh Young's modulus and low heat shrinkage.

Examples 18 and 19 and Comparative Examples 6 and 7

[0206] Six point five micrometers thick biaxially oriented polyesterfilms are obtained as described for Example 17 and Comparative Example5, except that polyethylene-2,6-naphthalate with an inherent viscosityof 0.65 (glass transition temperature 125° C., melting point 265° C.,containing 0.1 wt % of spherical crosslinked polystyrene particles withan average particle size of 0.3 μm) or a copolymer consisting of 90 mol% of ethylene terephthalate and 10 mol % of ethylene-2,6-naphthalate(glass transition temperature 84° C., melting point 235° C., containing0.1 wt % of spherical crosslinked polystyrene particles with an averageparticle size of 0.3 μm) is used, and that the stretching temperature isset as shown in Table 6. Also when polyethylene-2,6-naphthalate or saidcopolymer is used as the raw polymer, the effects of the small-ratiostretching by the present invention could be remarkably observed. Ifsimultaneous biaxial small-ratio stretching is effected repetitively,the total area stretching ratio and the degree of crystallinity can beenhanced, and a high quality polyester film with a high Young's modulusand low heat shrinkage can be stably produced. TABLE 6 Raw polyesterFilm forming conditions by simultaneous Young's Inherent biaxialstretching Total area modulas Heat shrinkage Degree of viscositySmall-ratio Temperature (° C.), stretching (GPa) (MD/TD) (%)crystallinity Composition (dl/g) stretching Stretching/Heat treatmentratio (MD/TD) (100° C., 30 min) (%) Example 17 PET 1.0 Effected 115/21058.0 8.6/8.4 0.6/0.8 43 Example 18 PEN 0.65 Effected 135/210 72.010.2/10.5 0.2/0.2 30 Example 19 PET/PEN (90/10) 0.65 Effected 120/21065.0 8.5/8.6 0.5/0.4 47 Comparative PET 1.0 Not effected 115/210 36.05.8/6.2 2.1/2.5 38 Example 5 Comparative PEN 0.65 Not effected 135/21049.0 7.2/7.4 0.3/0.4 24 Example 6 Comparative PET/PEN (90/10) 0.65 Noteffected 120/210 40.0 6.2/6.3 1.6/1.3 42 Example 7

Example 20 (Tables 7 and 8)

[0207] A cast film obtained as described for Example 1 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, and heated to a film temperature of100° C., and a series of operation consisting of simultaneous biaxialstretching at an area stretching ratio of 2.25 times (1.5 times in themachine direction and 1.5 times in the transverse direction) andsuccessive relaxation (at a relaxation rate of 5% in the machinedirection and 5% in the transverse direction) is effected 5 timescontinuously. Then, the film is heatset at 210° C., relaxed in a 120° C.cooling zone at a relaxation rate of 2% in the machine direction and 2%in the transverse direction, gradually cooled to room temperature, andwound. The film is controlled to achieve a thickness of 10 μm byadjusting the amount of extrusion. The clip temperature duringstretching is 100° C. The film obtained reached a total area stretchingratio of 34.5 times and had both a high Young's modulus and thermaldimensional stability, being a high quality film with little thicknessfluctuation. At the time of film formation, film breaking littleoccurred, and a film with high film properties and quality could be verystably obtained. If the stretching ratio and the repetition frequencyare the same as in Example 5 and relaxation treatment is added, then thethermal dimensional stability can be improved though the Young's modulusdeclines slightly.

Examples 21 to 23

[0208] Biaxially oriented polyester films are obtained as described forExample 20, except that the ratio of small-ratio stretching of eachtime, relaxation rate, repetition frequency and total area stretchingratio are changed. The obtained polyester films had both a high Young'smodulus and thermal dimensional stability, being high quality films withlittle thickness fluctuation as in Example 20. At the time of filmformation, film breaking little occurred, and films with high filmproperties and quality could be very stably obtained.

Example 24 (Tables 7 and 8)

[0209] A cast film obtained as described for Example 1 wassimultaneously biaxially stretched at a film temperature of 100° C. atan area stretching ratio of 16.0 times (4.0 times in the machinedirection and 4.0 times in the transverse direction) by a simultaneouslybiaxially stretching tenter oven, successively relaxed (at a relaxationrate of 5% in the machine direction and 5% in the transverse direction),simultaneously biaxially oriented at 170° C. at an area stretching ratioof 2.25 times (at a ratio of 1.5 times in the machine direction and 1.5times in the transverse direction), and in succession relaxed (at arelaxation rate of 5% in the machine direction and 5% in the transversedirection). Then, the film is heatset at 210° C., relaxed in a 120° C.cooling zone at a relaxation rate of 2% in the machine direction and 2%in the transverse direction, gradually cooled to room temperature, andwound. The obtained polyester film had both a high Young's modulus andthermal dimensional stability, being a high quality film with littlethickness fluctuation as in Example 20. At the time of film formation,film breaking little occurred, and a film with high film properties andquality could be very stably obtained.

Example 25

[0210] A cast film obtained as described for Example 1 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, and heated to a film temperature of100° C., and a series of operation consisting of simultaneous biaxialstretching at an area stretching ratio of 1.21 times (1.1 times in themachine direction and 1.1 times in the transverse direction) andsuccessive relaxation (at a relaxation rate of 5% in the machinedirection and 5% in the transverse direction) is effected 10 times.Then, simultaneous biaxial stretching at an area stretching ratio of1.21 times (1.1 times in the machine direction and 1.1 times in thetransverse direction) is effected twice, without relaxation. Insuccession, a series of operation consisting of simultaneous biaxialstretching at an area stretching ratio of 1.21 times (1.1 times in themachine direction and 1.1 times in the transverse direction) andsuccessive relaxation (at a relaxation rate of 5% in the machinedirection and 5% in the transverse direction) is effected 30 times. Thetotal repetition frequency of the series of operation consisting ofstretching and relaxation is 40 times. The total number of times ofstretching only is 2. Then, the film is heatset at 210° C., relaxed in a120° C. cooling zone at a relaxation rate of 2% in the machine directionand 2% in the transverse direction, gradually cooled to roomtemperature, and wound. The obtained film had both a high Young'smodulus and thermal dimensional stability, being a high quality filmwith little thickness fluctuation. At the time of film formation, filmbreaking little occurred, and a film with high film properties andquality could be very stably obtained.

Example 26

[0211] A cast film obtained as described for Example 1 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, and heated to a film temperature of100° C., and a series of operation consisting of simultaneous biaxialstretching at an area stretching ratio of 1.21 times (1.1 times in themachine direction and 1.1 times in the transverse direction) andsuccessive relaxation (at a relaxation rate of 5% in the machinedirection and 5% in the transverse direction) is effected 30 times.Then, relaxation (at a relation rate of 5% in the machine direction and5% in the transverse direction) and in succession a series of operationconsisting of similar simultaneous biaxial stretching at 140° C. andsuccessive relaxation is effected 5 times. Then, relaxation (at arelaxation rate of 5% in the machine direction and 5% in the transversedirection) is effected once, and in succession a series of operationconsisting of similar simultaneous biaxial stretching at 170° C. andsuccessive relaxation is effected 7 times. The total number of times ofthe series of operation consisting of stretching and relaxation is 42,and the total number of times of relaxation only is 2. Then, the film isheatset at 210° C., relaxed in a 120° C. cooling zone at a relaxationrate of 2% in the machine direction and 2% in the transverse direction,gradually cooled to room temperature, and wound. The obtained film hadboth a high Young's modulus and thermal dimensional stability, being ahigh quality film with little thickness fluctuation. At the time of filmformation, film breaking little occurred, and a film with high filmproperties and quality could be very stably obtained. TABLE 7 Stretchingratio, Relaxation rate Total Total longitudinal × lateral (longitudinal/repetition area (area ratio) (%) lateral) (%) frequency ratio Example 201.5 × 1.5(2.25) 5/5 5 34.5 Example 21 1.1 × 1.1(1.21) 5/5 45 52.5Example 22 1.9 × 1.9(3.61) 5/5 3 34.6 Example 23 1.5 × 1.5(2.25) 1/1 552.2 Example 24 (1)4.0 × 4.0(16.0)   5/5 1 29.3 (2)1.5 × 1.5(2.25)   5/51 Example 25 1.1 × 1.1(1.21) 5/5 40 49.5 1.1 × 1.1(1.21) None 2 Example26 1.1 × 1.1(1.21) 5/5 42 32.8 1.0 × 1.0(1.0)  5/5 2

[0212] TABLE 8 Young's Heat shrinkage Thickness modulus (GPa) (%)(MD/TD) Breaking fluctua- (MD/TD) (100° C., 30 min) frequency tion (%)Example 20 7.4/7.5 0.6/0.4 ⊚ 5 Example 21 8.4/8.6 0.4/0.2 ⊚ 4 Example 227.3/7.4 1.0/0.6 ◯ 7 Example 23 7.8/7.9 0.8/0.6 ◯ 8 Example 24 6.2/6.10.9/0.5 ◯ 7 Example 25 8.8/8.6 0.6/0.4 ⊚ 6 Example 26 8.3/8.5 0.4/0.2 ⊚6

Examples 27 and 28

[0213] Two extruders are used. Pellets of polyethylene terephthalate (I)(inherent viscosity 0.65, glass transition temperature 75° C., meltingtemperature 255° C., containing 0.16 wt % of spherical silica particleswith an average particle size of 0.07 μm) are dried in vacuum at 180° C.for 3hours, and supplied into extruder A heated to 280° C., and pelletsof polyethylene terephthalate (II) (inherent viscosity 0.65, glasstransition temperature 75° C., melting temperature 255° C., containing0.2 wt % of spherical crosslinked polystyrene particles with an averageparticle size of 0.3 μm and 0.01 wt % of spherical crosslinkedpolystyrene particles with an average particle size of 0.8 μm) are driedin vacuum at 180° C. for 3 hours, and supplied into extruder B alsoheated to 280° C. They are joined in a T die (lamination rationI/II=10/1), and the laminate is electrostatically brought into contactwith a casting drum with a surface temperature of 25° C., to be cooledand solidified for preparing a cast laminate film. In Example 27, thefilm is stretched as described for Example 1, and in Example 28, it isstretched as described for Example 20. The obtained 6.5 μm films areprocessed for magnetic recording media, and their practical propertiesas video tape and data tape are evaluated. They are found to haveexcellent properties as shown in Table 9.

Comparative Example 8

[0214] A cast film obtained as described for Example 27 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, heated to a film temperature of 95°C., simultaneously biaxially oriented to 3.5 times respectively in themachine and transverse directions, simultaneously biaxially stretched to1.3 times respectively in the machine and transverse directions at 110°C., heatset at 210° C., relaxed in a 120° C. cooling zone at arelaxation rate of 2% in the machine direction and 2% in the transversedirection, gradually cooled to room temperature, and wound, to obtain a6.5 μm thick film. The obtained film is processed for a magneticrecording medium. As shown in Table 9, the film is inferior to the filmsof the present invention in practical properties for a magneticrecording medium. TABLE 9 High speed Creep abrasion ElectromagneticRunning Storage compliance resistance conversion properties durabilitystability MD/TD(GPa⁻¹) Example 27 ◯ ◯ ◯ ◯ 0.22/0.21 Example 28 ◯ ◯ ◯ ◯0.23/0.22 Comparative X X Δ Δ 0.34/0.27 Example 8

Examples 29 and 30

[0215] Two extruders are used. Pellets of polyethylene terephthalate (I)(inherent viscosity 0.65, glass transition temperature 75° C., meltingtemperature 255° C., containing no particles) are dried in vacuum at180° C. for 3 hours and supplied into extruder A heated to 280° C., andpellets of polyethylene terephthalate (II) (inherent viscosity 0.65,glass transition temperature 75° C., melting temperature 255° C.,containing 6 wt % of spherical crosslinked polystyrene particles with anaverage particle size of 0.3 μm) are dried in vacuum at 180° C. for 3hours and supplied into extruder B heated to 310° C. They are joined ina T die (lamination ratio I/II=250/1), and the laminate iselectrostatically brought in contact with a casting drum with a surfacetemperature of 25° C., to be cooled and solidified, for preparing a castlaminate film. In Example 29, the film is stretched as described forExample 1, and in Example 30, the film is stretched as described forExample 20. The obtained 75 μm thick films are processed for magneticrecording media, and their practical properties as floppy discs areevaluated. They are found to have excellent properties as shown in Table10.

Comparative Example 9

[0216] A cast film obtained as described for Example 29 is held at boththe edges by clips, introduced into a linear motor driven simultaneouslybiaxially stretching tenter oven, heated to a film temperature of 95°C., simultaneously biaxially stretched to 4 times respectively in themachine and transverse directions, heatset at 210° C., relaxed in a 120°C. cooling zone at a relaxation rate of 2% in the machine direction and2% in the transverse direction, gradually cooled to room temperature,and wound, to obtain a 75 μm thick film. The obtained film is processedfor a floppy disc. Its practical properties for a floppy disc areinferior to the those of the films of the present invention as shown inTable 10. TABLE 10 Tracking resistance Change due to Change due toScratch temperature humidity resistance Example 29 ◯ ◯ ◯ Example 30 ◯ ◯◯ Comparative X X X Example 9

Examples 31 and 32

[0217] Pellets of polyethylene terephthalate (inherent viscosity 0.65,glass transition temperature 75° C., melting point 255° C., containing0.2 wt % of silicon dioxide particles with an average particle size of1.0 μm) are dried in vacuum at 180° C. for 3 hours, supplied into anextruder heated to 280° C., and melt-extruded from a T die as a sheet.The sheet is electrostatically brought into contact with a cooling drumwith a surface temperature of 25° C., to be cooled and solidified, forobtaining a cast film. A cast film is coated on one side with a coatingmaterial composed as follows by a gravure coater, to form a fusionpreventive layer with a dry thickness of 0.5 μm. (Composition of coatingmaterial) Acrylate 14.0 wt % Amino modified silicone  5.9 wt %Isocyanate  0.1 wt % Water 80.0 wt %

[0218] Then, using a simultaneously biaxially stretching tenter oven, inExample 31, the film is stretched as described for Example 1, and inExample 32, the film is stretched as described for Example 20. Theobtained 4 μm thick films are processed for heat transfer ribbons, andtheir practical property for heat transfer ribbons are evaluated. Theyare found to have an excellent property as shown in Table 11.

Comparative Example 10

[0219] A cast film with a fusion preventive layer on one side isobtained as described for Example 31. Then, using a simultaneouslybiaxially stretching tenter oven, the film is stretched as described forComparative Example 8, to obtain a 4 μm thick film for a heat transferribbon. The practical property of the film for a heat transfer ribbon isinferior to that of the films of the present invention as shown in Table11. TABLE 11 Printability Example 31 ◯ Example 32 ◯ Comparative Example10 X

Examples 33 and 34

[0220] Pellets of polyethylene terephthalate (inherent viscosity 0.65,glass transition temperature 75° C., melting point 255° C., containing0.1 wt % of agglomerate silica particles with an average particle sizeof 1.2 μm) are dried in vacuum at 180° C. for 3 hours, supplied into anextruder heated to 280° C., and melt-extruded from a T die as a sheet.The sheet is electrostatically brought into contact with a cooling drumwith a surface temperature of 25° C., to be cooled and solidified, forobtaining a cast film.

[0221] Then, using a simultaneously biaxially stretching tenter oven, inExample 33, the film is stretched as described for Example 1, and inExample 34, the film is stretched as described for Example 20. Theobtained 4 μm thick films are processed for capacitors, and theirpractical properties are evaluated. They are found to have excellentproperties as shown in Table 12.

Comparative Example 11

[0222] A cast film obtained as described for Example 33 is stretched asdescribed for Comparative Example 9, to obtain a 4 μm thick film. It isprocessed for a capacitor and its practical properties are evaluated. Asshown in Table 12, its practical properties for a capacitor are inferiorto those of the films of the present invention. TABLE 12 InsulationDielectric breakdown resistance voltage Example 33 ⊚ ◯ Example 34 ⊚ ◯Comparative Example 11 Δ X

Examples 35 and 36

[0223] Pellets of polyethylene terephthalate (inherent viscosity 0.65,glass transition temperature 75° C., melting point 255° C., containing0.4 wt % of agglomerate silica particles with an average particle sizeof 1.2 μm) are dried in vacuum at 180° C. for 3 hours, supplied into anextruder heated to 280° C., and melt-extruded from a T die as a sheet.The sheet is electrostatically brought into contact with a cooling drumwith a surface temperature of 25° C., to be cooled and solidified, forobtaining a cast film.

[0224] Then, using a simultaneously biaxially stretching tenter oven, inExample 35, the film is stretched as described for Example 1, and inExample 36, the film is stretched as described for Example 20. Theobtained 4 μm thick films are processed for thermal mimeographic stencilpaper, and their practical properties are evaluated. They are found tohave excellent properties as shown in Table 13.

Comparative Example 12

[0225] A cast film obtained as described for Example 35 is stretched asdescribed for Comparative Example 9, to obtain a 4 μm thick film. It isprocessed for a thermal mimeographic stencil paper, and its practicalproperties are evaluated. As shown in Table 13, its practical propertiesfor a thermal mimeographic stencil paper are inferior to those of thefilms of the present invention. TABLE 13 Voids Shade fluctuation Example35 ◯ ◯ Example 36 ◯ ◯ Comparative Example 12 X Δ

Industrial Applicability

[0226] According to the production method of the present invention, ahigh quality polyester film with a high rigidity, low heat shrinkage,less thickness fluctuation and less surface defects can be stablyproduced at a less breaking frequency. The present invention can bewidely applied as a method for producing various films for magneticrecording media, capacitors, heat transfer ribbons, thermal mimeographicstencil paper, packaging, etc., and the present invention can alsoprovide a new polyester film with properties and quality far moreexcellent than the mechanical properties of conventional polyesterfilms.

1. A method for producing a polyester film, in which a film made of aresin mainly composed of a polyester is stretched using a simultaneouslybiaxially stretching tenter oven, comprising the step of effecting aseries of operation consisting of stretching and subsequent relaxationtwice to less than 10000 times, to achieve a total area stretching ratioof 25 to 150 times.
 2. A method for producing a polyester film,according to claim 1, wherein the area stretching ratio by one time ofstretching in said stretching is 1.005 to 10 times and the relaxationrate in the relaxation is 0.1 to 80% based on the lengths in the machineand transverse directions reached immediately before relaxation.
 3. Apolyester film, produced by the method stated in claim
 1. 4. A polyesterfilm, produced by the method stated in claim
 2. 5. A polyester film,according to claim 3, wherein the sum of the Young's modulus in themachine and transverse directions of the film is 8 to 30 GPa, and thesum of heat shrinkage percentages at 100° C. for 30 minutes is 2% orless.
 6. A polyester film, according to claims 3, 4 or 5, wherein thedegree of crystallinity is 30 to 90%.
 7. A polyester film, according toany one of claims 3, 4 or 5, wherein the polyester is polyethyleneterephthalate, polyethylene naphthalate or their copolymer or modifiedpolymer.
 8. A polyester film, according to claim 6, wherein thepolyester is polyethylene terephthalate, polyethylene naphtalate ortheir copolymer or modified polymer.
 9. A polyester film, according toany one of claims 3, 4 or 5, wherein the inherent viscosity is 0.6 ormore.
 10. A polyester film, according to claim 6, wherein the inherentviscosity is 0.6 or more.
 11. A polyester film, according to claim 7,wherein the inherent viscosity is 0.6 or more.
 12. A magnetic recordingmedium, comprising a polyester film stated in any one of claims 3through
 5. 13. A magnetic recording medium, comprising a polyester filmas stated in claim
 6. 14. A magnetic recording medium, comprising apolyester film as stated in claim
 7. 15. A magnetic recording medium,comprising a polyester film as stated in claim
 8. 16. A magneticrecording medium, comprising a polyester film as stated in claim
 9. 17.A capacitor, comprising a polyester film stated in any one of claims 3through
 5. 18. A capacitor, comprising a polyester film as stated inclaim
 6. 19. A capacitor, comprising a polyester film as stated in claim7.
 20. A capacitor, comprising a polyester film as stated in claim 8.21. A capacitor, comprising a polyester film as stated in claim
 9. 22. Aheat transfer ribbon, comprising a polyester film stated in any one ofclaims 3 through
 5. 23. A heat transfer ribbon, comprising a polyesterfilm as stated in claim
 6. 24. A heat transfer ribbon, comprising apolyester film as stated in claim
 7. 25. A heat transfer ribbon,comprising a polyester film as stated in claim
 8. 26. A heat transferribbon, comprising a polyester film as stated in claim
 9. 27. A thermalmimeographic plate, comprising a polyester film stated in any one ofclaims 3 through
 5. 28. A thermal mimeographic plate, comprising apolyester film as stated in
 6. 29. A thermal mimeographic plate,comprising a polyester film as stated in
 7. 30. A thermal mimeographicplate, comprising a polyester film as stated in
 8. 31. A thermalmimeographic plate, comprising a polyester film as stated in 9.